1. Field of the Invention
The present invention relates to a variable lift valve operating system for an internal combustion engine, including: a variable lift mechanism which has a control shaft rotatably supported in a cylinder head, and which is capable of changing a lift amount of an engine valve in accordance with rotation of the control shaft; and an actuator which has an electric motor and power transmission means that is interposed between the electric motor and the control shaft, and which is connected to the control shaft.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2005-42642 discloses a valve operating system in which one end portion of a lever is fixed to a control shaft, and the control shaft is rotated by sliding a nut connected to the other end portion of the lever by the rotating operation of a screw shaft on which the nut is screwed, whereby the lift amount of an intake valve is changed.
In such a variable lift valve operating system, when an actuator has a structure which lubricates, with oil, power transmission means interposed between the electric motor and the control shaft, friction in the power transmission means changes due to change in oil viscosity depending on the ambient temperature, and therefore, it is difficult to rotationally drive the control shaft stably at all times.